A mechanism for rotation restraints in the knee joint.

Ligament function in restraining axial rotation of the tibia relative to the femur cannot be revealed by analysis of ligament forces alone. The action of the articular surfaces should be taken into account as well. In this study, three-dimensional mathematical models of four human knee joints were used to determine the limits of axial rotation between 0 and 90 degrees of flexion, whereby the forces in the ligaments and articular contact were calculated, together with their contribution to the restraint moment that was required to counterbalance the applied axial moment of 3 Nm. In external rotation, the direct axial restraint was provided by the collateral ligaments. In internal rotation, when the cruciate ligaments and medial collateral ligament were predominantly loaded, the direct restraint moment resulting from the ligament forces was not sufficient to counterbalance the applied moment. The articular contact forces, which resulted from balancing the axial components of the ligament forces, contributed considerably to the restraint of internal rotation. Depending on the flexion angle, the contact forces provided approximately 50-85% of the internal restraint, whereas 95-100% of the external rotation restraint was accounted for by the ligament forces.